As far as reomoving the rear deck, start by pulling up on the front of the seat part of the back seat. Then pull up and out to remove the seat back. You should see several trim fasteners holding the rear shelf in place remove those. The rear shelf should slide out. To remove the C pillar trim panels just grab them and gently pull them out. They're held in by tension fasteners. WARNING: the C pillar trim pieces area a hell to get back in place.
As far as installing a sub, are you thinking of just mounting the sub in the rear shelf? Unless the sub is a free-air sub (does not need an enclosure--relatively rare), I would recomend mounting it in a box and placing it in the trunk. Low frequencies will easily pass through the trunk walls and a sub in a box will get much louder much quicker than one that is not.